Light diffusing tip applicators find application in a number of clinical settings. Prevalent uses include the treatment of cancerous tumors using either photodynamic therapy (PDT) or laser interstitial thermal therapy (LITT). In PDT, light diffusing fiber optics are used to uniformly irradiate an organ or tissue that has been previously treated with a photo-sensitive light-activated compound which has been allowed to accumulate in the tumor. In LITT, laser energy is applied to tissues for treating solid malignant tumors in various organs such as the liver, brain, ear nose or throat (ENT), or abdominal tissues, as well as for treating benign alterations such as prostate adenomas. Volumetric heating within target tissues during LITT results in thermal tissue necrosis and tumor death.
Light diffusing tip applicators used to carry light from a source into a target tissue during such therapies can vary significantly in terms of their size and shape, as well as the way that they distribute light. A conventional bare fiber optic that terminates in a cleaved or polished face perpendicular to the optic axis is limited in most PDT and LITT procedures. To illustrate, for LITT procedures the power density and resulting heat generation using a bare fiber often exceed the thermal diffusion into the tissue, and areas close to the applicator therefore char or vaporize. These tissue phenomena are problematic for creating controlled photothermal lesions. For example, charring limits heat deposition within deeper tissue volumes due to increased absorption of light energy by the charred region. As charred tissue continues to absorb incident light, its temperature continues to rise leading to more carbonization around the applicator, and temperature rise in deeper layers is strictly dependent on heat conduction away from this carbonized volume. While it is possible to create large thermal lesions in this manner, the morphology of the resulting lesion is highly undesirable. Furthermore, the high temperatures associated with the carbonized tissue often result in burning and failure at the tip of the optical fiber with significant attendant risk for patients and equipment. Therefore, many LITT procedures employ an applicator with a light diffuser (or diffusing tip) at the delivery end of the optical fiber. In such applications, the scattering of light over a larger surface area provided by the diffusing tip reduces the power density on the adjacent tissue and creates a larger coagulation volume while minimizing char formation.
Several techniques have been developed to obtain scattering of light from an optical fiber. One conventional technique includes selecting the ratio of the index of refraction between the core of the optical fiber and the transparent cladding such that total internal reflection is prevented, thereby allowing light to escape and radiate outside of the fiber. It is difficult, however, to achieve uniform output intensity using this method, and its use therefore is not widespread. Other conventional techniques include etching the outer surface of the core or clad using chemical or mechanical means or embedding scattering particles around the outer surface of the core or within the cladding. Such techniques typically result in a decrease in the mechanical integrity of the fiber and frequently are incapable of achieving a wide range of light distributions.
Another conventional technique employs the use of a transmissive medium such as an epoxy with embedded scattering particles and a reflector at the tip. The reflector serves to both improve homogeneity of the light exiting the fiber as well as prevent significant forward light propagation. However, the use of metallic or dielectric reflectors or plugs limits the utility of such sensors because such reflectors may absorb light energy and lead to fiber failure. Moreover, metal reflectors, in particular, may not be compatible with new magnetic resonance imaging (MRI) image-guided procedures. A further disadvantage is that such reflectors may be difficult or expensive to produce. Finally, the reflector and scattering medium, being of significantly different materials with differing mechanical properties, may partially or fully separate at their interface, leading to potential “hot spots,” undesirable light distributions, or degradation of diffuser performance, all of which are likely to lead to a failure in the applicator.
Another conventional technique employs a cylindrical diffusing tip that includes an optically transparent core material such as silicone with scattering particles dispersed therein which abuts the core of the optical fiber. This diffusing tip is manufactured such that the concentration of scattering particles continuously increases from the proximal to distal ends of the diffusing tip. The increase in the concentration of scattering particles eliminates the need for a reflector because light is increasingly scattered along the diffusing tip length while the amount of light available decreases distally. However, this conventional technique has a number of limitations. For one, the gradient in the tip is extruded using a two-channel injector system with a mixing chamber whose contents are combined and extruded through a die. The contents are combined by varying the relative feed rates of elastomer with two different concentrations of scatterers to create a gradient in the scattering particles along the axial length of the diffusing tip. This mixing process places fundamental limitations on the range of gradients (e.g., the rate of change of said gradients) which can be produced. Moreover, this mixing process allows for the creation of gradients only in the direction of the axis of the fiber. A radial gradient in scattering particle concentration, for example, is unachievable by this conventional process.
Further, the elastomer-based tip is first extruded as described above, cut to length and then affixed to the end of the terminus face of the delivery fiber. A plastic tube then is slid over both the jacket of the optical fiber and the diffuser core. Thus the diffuser core must be separately affixed to the optical fiber core which results in a small bonding surface area. Further, an outer tube larger than the fiber's outer jacket is required, thereby increasing the overall diameter of the device beyond the outer diameter of the fiber's outer jacket. Another disadvantage related to affixing the tip in this manner is that there are no bonding interfaces to any circumferential surfaces of the fiber. The sole axial bond is vulnerable to defects such as air gaps, especially when flexion occurs at the interface between the optical fiber core and diffuser core that causes the two to separate. Air or other gaps between the optical fiber core and diffuser core change the intended light distribution and may result in unintended “hot spots” which significantly increase the risk of fiber failure during use. Gaps or defects in the interface between the diffusing core and the plastic tube placed over the core may also lead to “hot spots,” degradation of diffuser uniformity, and a decrease in power handling capability.
Accordingly, a light diffusing tip that overcomes the limitations of conventional light diffusing tips would be advantageous.